(1) Field of the Invention
The present invention relates to underwater weapons and more particularly, to directed energy high velocity jets used as an underwater weapon.
(2) Description of the Prior Art
As known in the art, undersea projectiles are considered a weapon to defeat undersea targets. Projectiles (similar to projectile 10 of FIG. 1), have been demonstrated for use. The projectiles are based on standard munitions with explosive cartridges launching the projectiles from a gun. Although the use of projectiles is an effective and low-risk approach for defeating underwater targets, the use presents a number of problems. In a first example, the launch system must be kept dry which further creates technical problems. In a second example of the problems of use, the combustion gasses produced by launch limit the rate of fire of the gun or weapon as these gasses interfere with flight of salvos of the projectiles 10. In a third example of the problems of use, the projectiles 10 interfere with each other in flight, further limiting rates of fire. In a final but not exhaustive example of the problems of use, the projectiles 10 occupy a very small portion of the supercavity 12 that they generate therefore utilizing a small percentage of the potential benefits of the supercavity 12.
It has been further demonstrated that forward-directed jets 20 from moving vehicles 22 (shown in FIG. 2) can produce supercavities 24 in a manner similar to a physical cavitator. As shown in the figure, the jet 20 advances forward of the vehicle 22 such that a moving front 26 is produced. The size and shape of the cavity 24 are related to the diameter of the forward directed jet 20 and the advancement speed of the moving front 26.
Referring again to FIG. 1, the shape of the cavity 12 is assumed to be elliptical as defined by                               (                                    x              -                              l                /                2                                                    l              /              2                                )                m            +                        (                      r            R                    )                n              =    1    ,where x is the distance along the axis of the cavity 12, l is the length of the cavity, r is the radius of the cavity, and R is the maximum radius of the cavity. The exponents are selected using the approximation as m=2 and n=2.4. Two other parameters are required to define the shape of the supercavity 12: λ(σ) and μ(σ, CD). CD is the cavitator drag coefficient based on the cavitator projected area and σ is the cavitation number defined as:   σ  =                    P        ∞            -              P        c                            1        /        2            ⁢                           ⁢      ρ      ⁢                           ⁢              U        2            where ρ is the fluid density, P∞ is the ambient pressure, PC is the pressure of the cavity 12, and U is the speed of the projectile 10. The first parameter, the ratio of the maximum diameter of the cavity 12 to cavitator tip diameter ratio is given by:   μ  =                              C          D                ⁡                  (                      1            +            σ                    )                            σ        ⁡                  (                      1            -                          0.132              ⁢                                                           ⁢                              σ                                  1                  /                  7                                                              )                    The second parameter, the slenderness ratio of the cavity 12, ½R, is given by:λ=1.067σ−0.658−0.52σ0.465 The drag coefficient of a disc cavitator is assumed equal to 0.814. An equivalence is assumed between a jet and a disc. A forward jet cavitator of known cross sectional area will produce a cavity equivalent in size and characteristics to a disc 20.5% of the size.
The required forward directed jet velocity can be estimated from energy balance considerations. The rate of work done by the jet front is the product of the drag force of the equivalent disc cavitator multiplied by the speed of advancement of the jet front, e.g.:      Power    out    =            1      2        ⁢          ρ      fluid        ⁢          U      f      2        ⁢          A      equiv        ⁢          C      d        ⁢          U      f      The energy flux into the jet front as supplied by the high-speed jet is computed relative to the advection speed of the front. This energy is then given by:      Power    in    =            (                        1          2                ⁢                                            ρ              jet                        ⁡                          (                                                U                  jet                                -                                  U                  f                                            )                                2                ⁢                  A          jet                    )        ⁢          (                        U          jet                -                  U          f                    )      
Setting these two expressions equal to each other provides a relationship between required jet velocities to sustain a propagating jet front as a function of a few key parameters:             ρ      fluid              ρ      jet        =                    A        jet                              A          equiv                ⁢                  C          d                      ⁢                  (                                            U              jet                        -                          U              f                                            U            f                          )            3      
If the density ratio is assumed equal to 1.0 (water jet into water), the area ratio is assume equal to 0.205, and the drag coefficient is equal to 0.814, the required jet velocity is 1.55 times the front advance speed. If high density jets are considered, the required jet velocity is somewhat lower, 1.28 for a specific gravity of 8.0. The extent of penetration of the jet for a given velocity is improved, but for a specified dynamic head, the penetration is considerably less. Inversely, a light liquid can be fired a range for a specified dynamic head.
Dynamics play an important role in the jet concept. A steady jet from a stationary platform cannot sustain a supercavity. The jet must be pulsed to reap the benefits of supercavitation.
FIG. 3 illustrates the transient nature of a pulsed supercavitating jet 30. It is assumed that the water jet emerges at its maximum speed Ujet. As soon as the jet begins (point 1), a front forms at the exit of a nozzle 32 and a supercavity is created. As fluid feeds the front from the left, the existing portion of the supercavity expands (point 2) and the jet front propagates to the right at Uf. After an amount of time, the parts of the supercavity originally formed by the start of the jet 30, collapse back onto the fluid stream (point 3). At this point in time the state of the system is an elliptical cavity with a core (point 4). The front continues to be fed by the jet 30 in the core of the supercavity and it proceeds to the right. Material in the core is consumed at the front until there is no longer any fluid inside the supercavity 30 (point 5). The supercavity 30 then collapses as the closure point catches up to the maximum penetration of the front (points 6 and 7).
The geometry of the jet 30 determines the total water consumed and range of the jet. The total penetration length is the length of the cavity plus the distance the trapped core can drive the front after the cavity closes. This extra length is simply determined as:       L    fp    =                    U        f            ⁢              L        cav                    (                        U          jet                -                  U          f                    )      
The total volume v of material consumed in forming the jet 30 is the volume in the core plus the fluid required to drive the front out to one length of the cavity from the nozzle 32.  V  =            A      jet        ⁡          (                        L          cav                +                              L            cav                    ⁢                                    U              jet                                      U              f                                          )      
In real world applications, high velocity jets are used in industrial systems for cutting operations. Pressures of 380 Mpa (50,000 psi), generated with specialized hydraulic pumps, and are used to generate very small diameter fluid jets with speeds approaching 800 m/s. These systems are designed for precision continuous cutting. As such, jet diameters are typically very small (no greater than 1 mm). Jet pulses of this size can only penetrate a very short distance (of the order 1 meter) in the water based on the equations described above. Power consumption for significantly larger jets becomes prohibitive if sustained operation is required.